Keratin fibers present in the granular layer of the epidermis aggregate by binding to a protein referred to as filaggrin during keratinization and produce a characteristic morphology referred to as a “keratin pattern”. Although a precursor substance of filaggrin known as profilaggrin (consisting of an arrangement of 10 to 12 filaggrin units) is present in large amounts in keratohyalin granules within granular cells, together with the formation of filaggrin monomers, keratin fibers are caused to aggregate by dephosphorylation during keratinization. Subsequently, the aggregated keratin fibers are subjected to deimination by the action of an enzyme known as peptidyl arginine deiminase (PAD), are released as keratin, and are subsequently decomposed to amino acids and the like in the upper layer of the horny layer. These amino acids are referred to as natural moisturizing factors (NMF), play an important role in maintaining the moisture content of the horny layer, and are known to possess the ability to absorb ultraviolet light (Blank, I. H., J.I. Dermatol., 18, 433 (1952); Blank, I. H., J.I. Dermatol., 21, 259 (1953)).
Ever since amino acids functioning as the main component of NMF were determined to originate in filaggrin, research has been conducted on the correlation between disease states presenting with dry skin and filaggrin. Amino acid levels in the horny layer have recently been determined to decrease in dry skin associated with conditions such as senile xerosis or atopic diseases (Horii, I. et al., Br. J. Dermatol., 121, 587-592 (1989); Tanaka, M. et al., Br. J. Dermatol., 139, 618-621 (1989)).
PAD deiminates filaggrin by acting on arginine residues, and converts them to citrulline residues. As a result of filaggrin being deiminated in this manner, the affinity between filaggrin and keratin fibers weakens and the keratin fibers are released, and as a result thereof, filaggrin becomes susceptible to the action of proteases, and this is ultimately thought to lead to its decomposition to NMF.
The inventor of the present invention identified calpain 1 as an enzyme that decomposes filaggrin deiminated by PAD, and determined that the decomposition products thereof in the form of small peptide fragments are decomposed to amino acid units, namely NMF, by bleomycin hydrolase (BH) (Journal of Investigative Dermatology (2008), Volume 128, Abstracts, S90, 539; Joint Conference of the 30th Annual Meeting of the Molecular Biology Society of Japan and 80th Annual Scientific Meeting of the Japanese Biochemical Society, Collection of Abstracts, p. 583; Journal of Biological Chemistry, 284, No. 19, pp. 12829-12836, 2009, 3P-0251; and, Japanese Patent Application No. 2008-135944 (to be referred to as JP944).
More recently, some atopic dermatitis is known to be caused by a genetic abnormality of the profilaggrin gene, and this genetic abnormality is observed in roughly 5% to 50% of atopic dermatitis patients (Smith, F. J. D., et al., Nat. Genet. 38: 337-342 (2006): Aileen Sandilands, et al., J.I. Dermatol., 127, 1282-1284 (2007); and, Nomura, T. et al., J.I. Dermatol., 128(6): 1436-41 (2008)). However, the skin of atopic dermatitis patients is not necessarily associated with a dramatic decrease in expression of filaggrin.